In recent years, air pollution caused by an automobile exhaust gas has become a serious problem and NOx, HC, CO, etc. of the exhaust gas have been restricted in quantities from a view point of preventing environmental pollution. The restriction is now getting more and more severe in consideration of acidic rain and others. Therefore, it is necessary to further improve an efficiency of the exhaust gas purification.
On the other hand, a recent increasing demand for a more powerful and capable engine tends to rise up the exhaust gas temperature. Under the circumstances, parts of an exhaust gas system are exposed to higher temperatures while driving the engine. Particularly, parts between the engine and a converter of exhaust gas purifying instruments, for example, an exhaust manifold, dual tube and the like, cannot help being exposed to still higher temperatures. In addition, these parts undergo not only changes in mechanical stress due to oscillation caused by driving the engine and running of the automobile, but also changes in temperature due to heating and cooling cycles depending upon patterns of driving and, in some cases, to freezing in cold areas. Thus, the parts are exposed to mechanically and thermally severe conditions.
As long as a heat resisting steel, for example, a stainless steel is applied as a material for the production of these parts, heat resistivity, of course, is excellent. However, because of weld-joints (the pipe used for these parts is usually made by weld and is often jointed to other parts by weld), the material must be excellent in weldability and in mechanical workability. Therefore, it is important that the material used for this purpose must be not only corrosion resistant which is the fundamental property of a stainless steel but also heat resistant, tough at low temperature, weldable and workable.
SUS304, a typical austinitic stainless steel, has been considered as a favorable material for use for the above-mentioned purpose because of its excellent workability and favorable weldability. However, since an austinitic stainless steel has a large thermal expansion coefficient, fears are entertained for a thermal fatigue cracking caused by a thermal stress which comes about in the repeated heating and cooling. In addition, because of a large difference in thermal expansion between an austinitic stainless steel and its surface oxide, the oxide layer tends to splinter off from the surface of the steel. For these reasons, a nickel base alloy represented by Inconel 600 is used in some parts as the pathway material for an exhaust gas of an automobile. This alloy is promising for the reasons that its thermal expansion coefficient is small whereby the oxide layer is tight adhesive to the surface and, in consequence, it is excellent in high temperature oxidation resistance as well as high temperature strength. However, this alloy is very expensive so that it is not extensively used.
On the other hand, when compared with the austinitic stainless steel, a ferritic stainless steel is cheap and, in addition, excellent in thermal fatigue properties because of its small thermal expansion coefficient, so that it is considered suitable for use in parts which are subjected to cyclic variation of temperature such as heating and cooling. Type 409 or SUS430, a representative of the ferritic stainless steel, is going on to use in part of an automobile exhaust gas path-way . However, these materials have a property that the strength goes sharply down as the temperature 900.degree. C. and higher, and in consequence, give rise to problems of which one is fatigue cracking due to insufficient strength and the other is abnormal oxidation when conditions go beyond the limit of oxidation resistivity. A counter action to these problems may be possible by means of addition of various alloying elements, which improve high temperature strength, or by means of increasing a chromium content to improve oxidation resistance. However, such means of addition of alloying elements or increase of chromium content make, in general, not only impact toughness of the steel weaken steeply but also weldability and workability get worse remarkably.
Any stainless steel that is in conformity with the above-mentioned conditions becoming more and more severe according to the demands for a more powerful and capable engine and for the progress of a purification efficiency of an exhaust gas is not come out yet. In other words, a material which is economical and satisfies simultaneously various demands for properties such as high temperature strength, oxidation resistance, heat resistance, toughness, weldability and workability is not yet obtainable from austinitic or the ferritic stainless steels nowadays. If a ferritic stainless steel retaining the previously stated desirable properties inherent to the ferritic stainless steel, and having improved heat resistivity and high temperature strength and, in addition, being excellent in productivity, workability, weldability and low temperature toughness comes to be obtainable, it may be said that such a material is very promising for the particular use mentioned above.
JP A 64-8254 discloses a ferritic stainless steel for the like use, but is completely silent with respect to low temperature toughness. JP B 59-52226 and 61-44121 disclose to improve a ferritic stainless steel in its rust development due to chlorine ion and its acid resistivity by adding copper and nickel while extremely lowering S, but teach nothing about high temperature strength, heat resistance, weldability and low temperature toughness.